Factorial Energy’s new deal with Mitsui Kinzoku is being reported as another solid-state partnership. It is better read as an admission: the cell is not the hard part. The sulfide solid electrolyte is — and Factorial has decided to buy it rather than make it.
Factorial and Japan’s Mitsui Kinzoku Company signed a joint development agreement on September 17 to industrialise Factorial’s Solstice all-solid-state platform. Under the split, Mitsui Kinzoku supplies the electrolyte and its manufacturing know-how; Factorial keeps cell design, process development, supply-chain qualification, line integration and manufacturing validation. Factorial is based in Billerica, Massachusetts, listed on Nasdaq in June 2026 under the ticker FAC, and does not build battery plants. It designs cells and validates how they get made.
The asymmetry is in what each side brings. Mitsui Kinzoku’s A-SOLiD is an argyrodite sulfide solid electrolyte first shown in 2016 and now heading for mass production at a dedicated plant in Saitama. The same company is the world’s leading producer of ultra-thin copper foil, a position it holds with an estimated 90% share of the semiconductor market for that material — a reminder that the solid-state supply chain runs through specialty materials firms, not car companies.
Why Sulfide Solid Electrolyte Decides the Timeline
Every solid-state battery programme in the world has converged on roughly the same physics: swap the flammable liquid electrolyte for a solid, and energy density and thermal stability both improve. Factorial quotes up to 450 Wh/kg for Solstice, roughly 80% above today’s lithium-ion cells, with stability to 90°C. Stellantis has measured 375 Wh/kg from Factorial’s 77 Ah FEST cells over more than 600 cycles, with 4C discharge, 15% to 90% charging in 18 minutes and operation from −30°C to 45°C.
The chemistry is not where these programmes die. The bottleneck is making the electrolyte — specifically sulfide electrolyte — at volume, at consistent quality, and at a price a car can carry. That is the constraint Factorial just outsourced.
The economics make it concrete. Lithium sulfide, the precursor that dominates sulfide electrolyte cost, trades above RMB 3 million per tonne (about $414,000 per tonne) in China, and accounts for more than half of the cell’s cost — against roughly 5% for the liquid electrolyte in a conventional lithium-ion cell. Industry forecasts put solid-state cell bill-of-materials at RMB 1–2 per Wh in early industrialisation ($0.14–0.28 per Wh), falling toward RMB 1 per Wh ($0.14) once annual deployment passes 10 GWh around 2030, and RMB 0.6–0.7 per Wh ($0.083–0.097) by 2035. Sulfide electrolyte demand is around 20 tonnes a year today and is projected to reach 94,000 tonnes by 2035, a market measured in the hundreds of billions of renminbi. Those are the numbers that decide whether 2027 is a launch date or a press release.
China’s Sulfide Solid Electrolyte Lines: Who Is Building What
This is where the Factorial deal looks narrow. Mitsui Kinzoku is one of very few producers outside China with sulfide electrolyte technology, and it is now the single materials partner behind a fabless American developer. In China, the same material is being chased by more than ten companies at once, from tonne-scale pilot lines to kilotonne construction plans:
| Company | Status reported | Next step |
|---|---|---|
| Enjie (Semcorp) | 10-tonne line in production (Yuxi, Yunnan) | 1,000 t/y lithium sulfide and sulfide halide electrolyte target by end-2027; stable batch production 2028–2030 |
| CAS Solid Energy | World’s first 100 t/y sulfide electrolyte line, ~300 kg/day | Full capacity expected 2025–2026 |
| Yanyi New Material | First 500 t/y line in stable, large-scale production | 1,000 t/y planned |
| Tianshi Kefeng (Wuhan) | 60 t/y powder line in production | 200 t by end-2026; 1,000 t by 2027 |
| Ruigu New Material | Phase 1 100 t line in production | Phase 2 kilotonne line accelerating |
| Yili Technology (Changzhou) | Phase 1 30 t in production | Expand toward 1,000 t |
| Tinci | Kilogram-scale pilot sampling | 100 t lithium sulfide and electrolyte pilot line expected Q3 2026 |
| Easpring | Sulfide small line plus 100 t oxide pilot built | Changzhou plan of 2,000 t oxide and 1,000 t sulfide electrolyte |
| Guanghua Technology | 300 t line built | 3000 t expansion prepared |
| Youyan New Material (GRIAM) | Kilogram-scale laboratory output | Tonnes in 2026 and a 1,000 t line by 2027; developing high-purity lithium sulfide with CATL since late 2024 |
| Others | BTR has sold sulfide-based materials since 2021; Kunlun, Guoci, Saike, Yinshi and Tian Shi run smaller lines | Ganfeng, Blue Solid and Qingtao are also positioned in the segment |
Two details in that table matter more than the headline capacities. The first is the engineering rhythm: Tianshi Kefeng, profiled by the Wuhan municipal investment bureau, replaced a 20-hour batch mixing process with a 20-minute one and moved from furnaces producing 3 kg per run to 100 kg, with 500 kg and larger as the next targets. Consistency at volume is the whole game, and that is what those numbers describe. The second is vertical integration: Youyan is developing high-purity lithium sulfide with CATL, and BYD builds its own cells — so the Chinese response to the electrolyte bottleneck is not a partnership, it is ownership.
Factorial’s Fabless Bet Against the 2027 Timetable
Factorial’s partner list is real. Mercedes-Benz ran a modified EQS for more than 1,200 km on a single charge using 106 Factorial cells in September 2025. Stellantis has put FEST cells into a Dodge Charger Daytona prototype now testing on North American roads, with field telemetry expected in early 2027. Hyundai is a partner, SK On signed a memorandum of understanding in July 2026 to bring Factorial’s cell technology into its manufacturing network, and Factorial booked its first commercial passenger-vehicle orders in January 2026 and its first aerospace order in July.
The company’s own framing is that it holds the cell design and the process, and lets partners hold the factories. Its technology is described as 80% drop-in compatible with existing lithium-ion lines, with a proprietary dry-cathode process that removes liquid solvents from cell assembly — a cost and factory argument, not just a chemistry argument.
Against that, the Chinese roadmap is more explicit about dates. Academician Ouyang Minggao’s three-generation framing puts first-generation sulfide all-solid-state cells at 200–300 Wh/kg in 2025–2027, second-generation high-silicon cells at 400 Wh/kg and 800 Wh/L in 2027–2030, and lithium-metal cells at 500 Wh/kg and 1,000 Wh/L in 2030–2035. Chinese brokerage research (Guojin Securities) targets 400 Wh/kg with more than 1,000 cycles, small-batch sedan fitment in 2027 and volume production in 2030. GAC has said it will install all-solid-state cells in 2026; Changan targets gradual volume production in 2027; BYD plans to put its own 400 Wh/kg sulfide all-solid-state cell into the Yangwang U9 in small volumes in 2027.
So both sides of the Pacific are pointing at 2027. The difference is what sits behind the date. Factorial’s 2027 depends on a cell it designs, a process it validates, and an electrolyte made by one Japanese supplier at a plant that is still described as heading toward mass production. BYD’s 2027 depends on a cell it makes, in a car it builds, using material it is developing with a domestic supply chain. Those are not the same kind of promise.
What the Factorial–Mitsui Kinzoku Deal Does Not Tell Us
Four limits are worth stating plainly.
A joint development agreement is not capacity. Nothing published specifies volumes, pricing, exclusivity, or when Mitsui Kinzoku’s Saitama plant reaches commercial output. “Advancing toward mass production” is not a date.
The 450 Wh/kg figure is a platform target. It is Factorial’s own number for Solstice, not a validated production specification, and it sits alongside 375 Wh/kg measured on the older FEST cells. The gap between a platform claim and a shipping cell is where solid-state programmes have historically slipped.
The cost figures come from the Chinese market. The RMB 3 million per tonne lithium sulfide price, the more-than-50% share of cell cost and the demand projections are Chinese industry and brokerage estimates. They describe the economics China is working against; they are the right order of magnitude for the material globally, but they are not Factorial’s or Mitsui Kinzoku’s numbers.
Capacity plans are announcements. The table above is compiled from Chinese industry surveys and official local-government reporting. We have not verified every project on site, and several entries are construction plans rather than operating lines. What is not in doubt is the count: more than ten companies, most of them unknown outside China, are building the same material.
Accuracy note: Factorial Energy and Mitsui Kinzoku Company signed a joint development agreement announced on September 17, 2026, covering industrialisation of Factorial’s Solstice sulfide-based all-solid-state platform. Product figures — 450 Wh/kg for Solstice, thermal stability to 90°C, the 80% drop-in compatibility claim, the dry-cathode process, and the FEST cell results of 375 Wh/kg over more than 600 cycles, 4C discharge and 15–90% charging in 18 minutes — are company statements, several of them published by Stellantis rather than by an independent laboratory. The 1,200 km single-charge run was a modified Mercedes-Benz EQS test vehicle in September 2025. Currency conversions use RMB 7.25 per USD, consistent with EVsays’ other coverage. Cost, demand and cell bill-of-materials figures for sulfide electrolyte and lithium sulfide are Chinese industry and brokerage estimates as reported in the Chinese trade press, not manufacturer disclosures; the 2035 demand projection is a forecast, not an order book. The capacity table is compiled from a Chinese-language industry survey of sulfide electrolyte capacity plans and from provincial government reporting, and individual entries have not been independently verified by us. Mitsui Kinzoku’s roughly 90% share applies to ultra-thin copper foil for semiconductors and should not be read as a battery market share. Neither company disclosed volumes, pricing or a commercial start date for electrolyte supply.
Sourcing note: The primary sources are Mitsui Kinzoku’s own technical page for its A-SOLiD sulfide electrolyte and the companies’ joint announcement as reported by the trade press on September 17, 2026. Factorial’s platform specifications, partner programme results and corporate timeline come from its own materials and from its partners’ releases. The electrolyte cost, capacity and demand data come from Chinese-language industry surveys, brokerage research and provincial government reporting on individual projects; where a company position is cited, it is as that company has stated it. EVsays did not attend any related event and has no access to either company beyond published materials, so every performance figure here is attributed to its origin rather than presented as verified fact. The assessment of what the deal does and does not establish is ours. Our editorial standards are set out in our editorial policy, and corrections are handled under our correction policy.
Sources & Further Reading
- Mitsui Kinzoku — A-SOLiD sulfide solid electrolyte (official technical page) — the electrolyte at the centre of the agreement, its argyrodite chemistry and the company’s position in sulfide solid electrolytes and ultra-thin copper foil.
- Electrek — “Solid-state battery maker Factorial secures another big partnership” (Peter Johnson, 2026-09-17) — the agreement itself, Factorial’s Solstice and FEST specifications, the Mercedes-Benz EQS and Stellantis Dodge Charger programmes, the SK On memorandum, and CEO Siyu Huang’s remarks.
- Wuhan Municipal Investment Promotion Bureau — profile of Tianshi Kefeng’s sulfide solid electrolyte plant (2026-07-20) — the 60 t/y line, the mixing-time and furnace-capacity gains, and the company’s expansion targets.
- Chinese industry survey of sulfide solid electrolyte capacity plans — the company-by-company position table, including Enjie, CAS Solid Energy, Yanyi, Ruigu, Yili, Tinci, Easpring, Guanghua and Youyan.
- Chinese brokerage research (Guojin Securities, Everbright Securities) as reported in the Chinese trade press — lithium sulfide pricing, the share of cell cost, cell bill-of-materials forecasts to 2035, and the 2027 small-batch / 2030 volume production timeline.
- EVsays — related coverage: the 2027 solid-state production race, FAW’s solid-state pilot line, Japanese electrolyte additive work, ProLogium’s production plans and BYD’s solid-state demonstration.
